Questions tagged [brdf]
136 questions
44
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2 answers
What is Importance Sampling?
What is importance sampling? Every article I read about it mentions 'PDF' what is that as well?
From what I gather, importance sampling is a technique to only sample areas on a hemisphere that matter more than others. So, ideally, I should sample…
Arjan Singh
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36
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How physically-based is the diffuse and specular distinction?
The classical way of shading surfaces in real-time computer graphics is a combination of a (Lambertian) diffuse term and a specular term, most likely Phong or Blinn-Phong.
Now with the trend going towards physically-based rendering and thus…
David Kuri
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30
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Path tracing the Cook-Torrance BRDF
-- Sorry for the long post, but I prefer to do that way because "Devil is in the details." :)
I am writing a path tracer from the scratch and it is working nicely for perfectly diffuse (Lambertian) surfaces (i.e. the furnace test indicates - at…
user5681
23
votes
3 answers
What is the difference between radiance and irradiance in BRDF
Currently I am reading the BRDF section from Real Time Rendering and I am having a hard time to visually understand the definition of this function. BRDF is the ratio between the radiance in outgoing direction and irradiance of incoming direction. I…
kevinyu
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22
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2 answers
How to properly combine the diffuse and specular terms?
As far as I understand, in a BRDF the Fresnel term is telling us the probability for a photon to be reflected or refracted when it hits a surface.
The reflected photons will contribute to the specular term, while the refracted ones will contribute…
Julien Guertault
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18
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2 answers
What is the accepted method of converting shininess to roughness, and vice versa?
Most modern renderers use physically-based materials and their models are often parameterized over roughness. Since this wasn't always the case with renderers, conventional assets often don't have a notion of roughness. Instead, we see "shininess"…
ap_
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16
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Fresnel reflectance of gold: red channel greater than 1?
I've been trying to understand some of the physical principles behind light and material interaction lately. In his talk Physics and Math of Shading, Naty Hoffman explains Fresnel reflectance and defines the characteristic specular color F0 of a…
David Kuri
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15
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1 answer
Choosing Reflection or Refraction in Path Tracing
I am trying to implement refraction and transmission in my path tracer and I'm a bit unsure on how to implement it. First, some background:
When light hits a surface, a portion of it will reflect, and a portion will be refracted:
How much light…
RichieSams
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14
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1 answer
Why isn't a BRDF a ratio of radiances?
I'm Learning about BRDFs and wondering why the BRDF is defined as the ratio of outgoing radiance to a given direction and incoming irradiance from another direction. Why isn't the BRDF defined as ratio of radiances?
PeteUK
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13
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2 answers
In a physically based BRDF, what vector should be used to compute the Fresnel coefficient?
The well known Schlick approximation of the Fresnel coefficient gives the equation:
$F=F_0+(1 - F_0)(1 - cos(\theta))^5$
And $cos(\theta)$ is equal to the dot product of the surface normal vector and the view vector.
It is still unclear to me though…
Julien Guertault
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12
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1 answer
Correct form of the GGX geometry term
I'm trying to implement a microfacet BRDF in my raytracer but I'm running into some issues. A lot of the papers and articles I've read define the partial geometry term as a function of the view and half vectors: G1(v, h). However, when implementing…
Erwin
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11
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1 answer
Compensation for energy loss in single-scattering microfacet BSDF models
Single-scattering microfacet-based surface models like the original Torrance-Sparrow BRDF or derived models like the BSDF for rough dielectric surfaces by Walter et al. neglect inter-reflection of light between microfacets, which results in energy…
ivokabel
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10
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2 answers
Why normalise Lambertian BRDF by 1/pi?
Why is a Lambertian BRDF normalised by dividing by $\pi$? Since the area of a unit sphere is $4 \pi$, and the area of the half sphere above the surface is $2 \pi$, shouldn't it rather be $1/(2\pi)$?
Supernormal
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10
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Rendering equation - why unsolvable directly?
Why is the rendering equation, introduced by Kajiya in 1986, not solvable directly/analytically?
Blongphong
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10
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1 answer
Reasons of the assumptions for the microfacet distribution function?
The paper Microfacet Models for Refraction through Rough Surfaces (among others) reminds us the following assumptions about the microfacet distribution function D:
Microfacet density is positive valued
Total microsurface area is at least as large…
wip
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